Relationship between Hydrocarbon Gas Generation and Kerogen Structural Evolution Revealed by Closed System Pyrolysis and Quantitative Py-GC Analysis of a Type II Kerogen

Kerogen can release both liquid and gaseous hydrocarbons during thermal maturation. In this study, the maturation and hydrocarbon generation of a low-maturity type II kerogen were simulated via temperature-programmed gold tube closed system (GTCS) pyrolysis at two different heating rates. In addition to the yields of C1–C5 gaseous hydrocarbons, H/C atomic ratios and δ13C values were also obtained on the kerogen residues from GTCS pyrolyses. The remaining C1–C5 gaseous hydrocarbon and C6+ liquid hydrocarbon generation potentials in both the original kerogen and the kerogen residues of various thermal maturities were determined by quantitative flash pyrolysis-gas chromatography (Py-GC) and were used to estimate the loss of aliphatic carbons in artificially matured kerogens compared with that in the original kerogen. The yields of C1–C5 gaseous hydrocarbons from GTCS pyrolysis were compared with the lost amount of C1–C5 gaseous hydrocarbon potentials within the kerogen structure at the same maturities. The results indicate that the H/C atomic ratios of kerogen residues decreased and the δ13C values of kerogen residues became progressively heavier (i.e., being enriched in 13C) with increasing maturity in the oil generation window (OGW) but with δ13C becoming slightly lighter above the calculated Ro (Calcd Ro, namely, Easy Ro or equivalent vitrinite reflectance) of 2.5%. It is worth noting that, in the OGW, the yields of C1–C5 gaseous hydrocarbons from GTCS pyrolyses were significantly lower than the amounts of lost C1–C5 potentials indicated by Py-GC. We speculate that hydrocarbon gases generated in OGW may either be partially adsorbed within kerogen and dissolved in oil or participate in the thermal degradation of kerogen during GTCS pyrolyses, resulting in lower yields of C1–C5 gaseous hydrocarbons than the decrease in kerogen C1–C5 potentials. The Py-GC results of kerogen residues also show that the rates of loss for C1, C2, and C3 hydrocarbons are about 92%, 98%, and 99%, respectively, at 1.3% Calcd Ro. This indicates that most of the C2+ alkyl side chains can be cleaved from the kerogen structure within OGW. In comparison, at a Calcd Ro of 1.3%, the formation rate of methane (C1) from GTCS pyrolysis of kerogen at a heating rate of 2 °C/h only amounted to about 32%. The results from this study appear to suggest that, in GTCS pyrolysis, most C1 generated from type II kerogen at the overmature stage cannot be attributed to primary kerogen cracking but to gradual secondary cracking of hydrocarbons with high molecular weights.